LED calibration standards today use an LED which uses 20 mA of current. An example of a calibration standard today is the Inphora IPR-DES2 unit. This unit supplies about 20 mA at approximately 3V to the LED. A key attribute in a stable calibration standard is the temperature of the LED is controlled. In the Inphora IPR-DES2 unit this is done by sensing the LED voltage. The LED voltage will change depending on temperature approximately −2 mV per degree C. This is fed back to the circuit to stabilize the temperature.
LEDs will change properties over time. Observations have been made where the voltage across the LED will increase over time, even though the temperature and current are controlled. The variation over time is likely due to changes in the optically active band gap of the semiconductor. Thus a feedback system using the LED voltage will vary the controlled temperature. The rate of change of this property varies on how long the LED has been in operation. One technique to minimize this effect is to age the LED until the rate of change is low enough to tolerate.
In the Inphora IPR-DES2 unit, the LED has a power dissipation of 60 mW. The low power dissipated is adequately cooled with the surrounding environment. Since the LED temperature is above ambient, heating is the main issue, not cooling. For HBLED units the power dissipation is 3.2 W or higher; therefore more power needs to be dissipated. Thus the cooling and dissipation of the heat would be a problem with the current scheme.
The LED junction is maintained at a higher temperature than the surrounding environment. Nominally 60 degrees C. is the target temperature in the Inphora IPR-DES2 unit. By picking a temperature higher than ambient, only a heater is needed to control the temperature of the LED. The Inphora IPR-DES2 unit currently has a heater, but has no need for heat removal given the low power dissipated.
Therefore what is needed is an LED calibration standard that can dissipate the greater amount of heat generated by HBLED and provide a controlled HBLED light output that is independent of the age of the HBLED used in the standard.